1. Field of the Invention
The present invention relates to a piezoelectric filter with low insertion loss for use in a radio-frequency (RF, particularly, GHz or higher-frequency) stage in a communication device such as a cellular telephone, and also relates to an electronic component including such a piezoelectric filter, such as a duplexer.
2. Description of the Related Art
Recently, there have been developed radio-frequency (RF, particularly, GHz or higher-frequency) stage filters for use in communication devices such as cellular telephones, which use a piezoelectric resonator having desirable characteristics. Specifically, such a resonator is compact and lightweight, and has high resistance to vibration or shock, as well as high reliability with less product variation. In addition, the resonator is free from circuit regulation, thus allowing for automation and simplification of mounting, and can be easily manufactured even when it is designed for the high-frequency environment.
Such a piezoelectric resonator may have electrodes disposed on both surfaces of a piezoelectric substrate, and may use thickness longitudinal vibration or thickness shear vibration of the piezoelectric substrate between the electrodes. The resonant frequency of the piezoelectric resonator using thickness longitudinal vibration of the piezoelectric substrate is inversely proportional to the thickness of the piezoelectric substrate. In a very high frequency region, therefore, the piezoelectric substrate must be extremely thin.
In an approach to thin a piezoelectric substrate itself, however, there is a high-frequency limit of several hundred megahertz in practice, in a fundamental mode, due to limitations on mechanical strength or handling. In order to solve such a high-frequency limitation problem, a piezoelectric thin film resonator used as a filter or resonator is described in, for example, Japanese Unexamined Patent Application Publication No. 2001-168674 published on Jun. 22, 2001.
In this piezoelectric thin film resonator, a thin film supporting portion can be made thin using micromachining technology, and a piezoelectric thin film can also be made thin by a technique such as sputtering. Therefore, the high-frequency characteristic of the piezoelectric thin film resonator can be extended to as high as several hundred megahertz to several thousand megahertz.
In order to improve the resonant-frequency temperature characteristic, a resonator combined with a SiO2 thin film having a positive resonant-frequency temperature coefficient is described in Japanese Unexamined Patent Application Publication No. 58-121817 published on Jul. 20, 1983 and Japanese Unexamined Patent Application Publication No. 58-137317 published on Aug. 15, 1983.
Also, a piezoelectric resonator including a bottom electrode, a piezoelectric film made of aluminum nitride (AlN), and a top electrode, wherein the electrode material is molybdenum (Mo) having a low thermo-elastic loss in order to increase the Q factor of the piezoelectric resonator is described in Japanese Unexamined Patent Application Publication No. 2000-69594 published on Mar. 3, 2000.
When any of the above-mentioned piezoelectric resonators is used to form a ladder filter with low insertion loss, the resonant frequency of a series piezoelectric resonator and the resonant frequency of a parallel piezoelectric resonator must be shifted substantially by a frequency corresponding to the passband of each other. In general, the frequency of the parallel piezoelectric resonator must be lowered.
One frequency shifting method, as described in, for example, Japanese Unexamined Patent Application Publication No. 58-121815, is to deposit a layer on the backside of a diaphragm using a technique such as evaporation so that this mass addition effect is utilized for frequency shifting. Another method is to add a layer on the top surface of a specific piezoelectric resonator and to remove a portion of the layer so as to shift the resonant frequency to the desired resonant frequency, as described in the U.S. Pat. No. 5,894,647 issued on Apr. 20, 1999.
In a filter formed of a plurality of piezoelectric resonators having different frequency characteristics (passbands), such as a ladder filter or a multimode filter, the thicknesses of vibrating portions each being disposed of the bottom electrode, the piezoelectric thin film, the top electrode, and so on differ from each other, resulting in different frequency characteristics (passbands).
Although the bottom electrodes, the piezoelectric thin films, or insulating films which may be formed beneath the bottom electrodes in some cases can have different thicknesses, the number of production steps and the production costs increase.
Accordingly, there has been proposed an approach in which the thicknesses of the bottom electrodes, the piezoelectric thin films, or the insulating films are common to some extent and the top electrodes, which are made of the same material, have different thicknesses from one piezoelectric resonator to another, resulting in different frequency characteristics (passbands).
However, the piezoelectric resonators require desirable film ratios depending upon the frequency characteristics (passbands). If one of the piezoelectric resonators having different frequency characteristics (passbands) includes a top electrode fabricated with the optimum film configuration, the other (or another) piezoelectric resonator cannot accomplish the desirable film configuration.